1. Field of the Disclosure
Embodiments disclosed herein relate generally to an open-loop refrigeration system for chilling ethylene to required storage temperatures. More specifically, embodiments disclosed herein relate to an integrated process for converting methanol to olefins (MTO), where the ethylene product from the MTO process is chilled to storage temperatures using an open-loop refrigeration system.
2. Background
Limited availability and high cost petroleum sources have led to the increased cost of producing basic commodity chemicals and their derivatives from such petroleum sources. As a result, various alternative competing technologies have been developed and commercially implemented in order to produce these chemicals from non-petroleum sources at a competitive cost.
One such technology involves catalytically converting methanol to olefins (MTO). Methanol is a readily available feedstock, which can be manufactured both from petroleum as well as from non-petroleum sources, for example, by fermentation of biomass or from synthesis gas.
A typical MTO process, as disclosed in U.S. Pat. No. 4,499,327, which is hereby incorporated in its entirety, involves contacting methanol with a zeolite catalyst, such as aluminosilicate, under conditions of temperature and pressure in order to produce olefins, such as ethylene. Ethylene is an extremely valuable commodity chemical for producing various chemical derivatives and polymers used in many commercial as well as consumer products and applications.
Before ethylene produced by an MTO process can be sold or used, it is necessary to recover the ethylene component in a desirable, ethylene-rich fraction by separating it from other components and impurities. For example, depending on the feedstock composition, the reaction conditions, and the extent of side reactions, an MTO effluent can contain other light olefins, diolefins and paraffins, such as methane, ethane, and propane.
One process for the separating and recovering of ethylene from an MTO process effluent involves the use of flash stages and distillation at cryogenic temperatures, as described in U.S. Pat. Nos. 7,166,757 and 4,499,327. For example, ethylene may be recovered from methane using cryogenic boiling point separation at temperatures that may be less than −90° C. Such cryogenic processes typically use closed-loop refrigeration systems, as described in U.S. Pat. No. 4,167,402, whereby a refrigerant, such as ethylene or methane, is circulated in a loop to indirectly chill the process gas, such as the MTO effluent.
The cryogenic separation can be very expensive due to both the capital cost of the specialized vessel metallurgy and refrigeration equipment, and the operating costs, including compression and cooling for the energy-intensive chill train. Additionally, closed-loop refrigeration systems may require a large inventory of a specialized refrigerant fluid. Due to these and other concerns, some MTO processes use non-cryogenic alternative methods for separation and recovery of ethylene from an MTO effluent.
One such alternative method for separating and recovering the ethylene from an MTO effluent involves physical absorption. For example, as described in U.S. patent application Ser. No. 12/260,751, which is incorporated herein by reference, an extractive distillation process using a physical absorbent, such as a C2-C4 hydrocarbon, may be used to concurrently absorb ethylene from a mixture of ethylene and methane, and to separate the methane from the ethylene and the absorbent at non-cryogenic temperatures of higher than approximately −40° C.
One trade-off of using a non-cryogenic method for separating the MTO effluent to recover an ethylene-rich fraction is that the resulting ethylene-rich fraction must either be stored as a gas at a high pressure or refrigerated to allow for atmospheric storage as a cryogenic liquid. The high-pressure gas storage option may require high capital costs, including high-pressure heavy-wall spherical storage vessels, and high compression costs. In the alternative, using a closed-loop refrigeration system to subcool the ethylene-rich fraction for atmospheric storage may be expensive, including the cost of an additional refrigeration compressor. Further, the cryogenic temperatures achieved via closed-loop refrigeration may not be low enough to prevent ethylene flashing in the atmospheric storage tank and require the use of tank re-compressors.